From the early days of projection TV, the picture projector was formed of three cathode ray tubes each displaying with high intensity in a single primary color phosphor, a picture for the respective colour received by the corresponding cathode ray tube. The three pictures were projected and aligned via an optical lens system. The resulting pictures were often poorly focused and the colours were often misaligned. Furthermore, after aging the misalignment became worse, requiring frequent servicing. Because of the requirement to use high intensity cathode ray tubes, which were driven close to their maximum tolerance, the lifetime of the cathode ray tubes was short. Therefore the operation of a projection TV was costly due to the requirement for replacement parts and due to the requirement for frequent servicing.
Other techniques have evolved to eliminate the requirement for use of high intensity cathode ray tubes. These generally utilize a light source such as a halogen or xenon lamp, filters to separate the various primary colours, and liquid crystals to form the pixel elements from each of the primary colours of light. Such systems are described for instance in U.S. Pat Nos. 4,936,658 issued Jun. 26, 1990, 4,861,142 issued Aug. 29, 1989 and 4,796,978 issued Jan. 10, 1989 to Seikosha Co., Ltd., U.S. Pat Nos. 5,073,013 issued Dec. 17, 1991, 4,904,061 issued Feb. 27, 1990 and 4,995,702 issued Feb. 26, 1991 to Seiko Epson Corporation.
The systems in these patents all suffer from one or more several problems. For example, the distance of travel of each colour of light to the final projection lens after being formed into a picture is different. This requires the use of spherical mirrors prior to the mirror combining system, resulting in fringing due to convergence problems.
These problems are compounded as the device heats during use and ages, or due to vibration during transportation causing minute movements of the elements forming the projection system.
In addition, the index of refraction for each of the couplers (filters) in each of the transmission elements within the projection structure is different for different colours, and even if they were not different, the refraction would vary because of the different distances the different coloured light beams are required to take. This also causes fringing, with deterioration in the resulting picture.